1. Field of the Invention
This invention relates to a cathode-ray tube device and more particularly to shielding an alternating electric field radiated from a deflection yoke of the cathode-ray tube device.
2. Description of the Related Art
A conventional cathode-ray tube device is described with reference to FIGS. 8 to 10.
FIG. 9 is a drawing of the main unit of a conventional cathode-ray tube, wherein a funnel 1 consists of a neck section 1a, a cone section 1b, and a main funnel section 1c having a high voltage anode button 1d. The joint of the neck section 1a and the cone section 1b is called a neck seal line 1e. A face panel 2 is jointed to the front of the funnel 1 by using frit glass. A glass bulb 20 is thus formed.
An electron gun 3 is sealed inside the neck section 1a. An explosion-proof band 4 is wound around the perimeter of the face panel 2 to guarantee an explosion-proof characteristic, and ears 4a are provided at the four corners of the face panel 2 to connect the glass bulb 20 to a chassis (not shown) for grounding. Further, a silicon resin film 5 for insulation is formed around the high voltage anode button 1d disposed in the funnel main section 1c. A conductive film 6 for addition of capacitance is disposed on the perimeter of the main funnel section 1c. Normally, the conductive film 6 is formed by applying graphite. Numeral 28 is a straight line parallel to the neck section 1a and shows the tube axis of the cathode-ray tube.
The cathode-ray tube thus formed is provided with a deflection yoke to deflect an electron beam at a position between the cone section 1b and the neck section 1a, as shown in FIG. 8. The deflection yoke 7 consists of a horizontal deflecting coil 7a, a vertical deflecting coil 7b, and a deflection yoke main section 7c.
In operation, when an electron beam is emitted from the electron gun 8 sealed in the neck section 1a, the electron beam is deflected a predetermined amount in the horizontal and vertical directions by the horizontal deflecting coil 7a and the vertical deflecting coil 7b of the deflection yoke 7 and scans on a fluorescent film formed on the inner face of the face panel 2 for displaying a desired image.
FIG. 11 shows another conventional example. A transparent conductive film 21 is disposed on the outer surface of a face panel 2 of the cathode-ray tube. The conductive film 21 is connected to an explosion-proof band 4 and further to a conductive film 6 for addition of capacitance by a conduction tape 18. Since the conductive film 6 for addition of capacitance is grounded through ground wire 29, the potential of the conductive film 21 is also zero volts.
The conductive film 21 is provided to prevent charging on the surface of the face panel 2, to suppress glare of reflection, etc., and to reduce electronic waves radiated forward from the deflection yoke 7 (Japanese Patent Lain-Open Nos.Sho 61-101950 and Hei 3-131180).
The deflection yoke generates an alternating field so as to cause an electron beam to scan on the inner surface of the face panel of the cathode-ray tube; at the same time, it also radiates an alternating electric field.
In recent years, the harmful effects of an electro-magnetic wave to human bodies have been considered an issue and with respect to display monitors, tile harmful effects of an alternating field emitted mainly from a deflecting yoke to the human bodies have also been feared. From such viewpoints, in 1991, MPR (National Council for Netrelogy and Testing), TCO (Central Organization of Salaried Employees in Sweden), etc., proposed standards for allowable electro-magnetic waves radiated from display monitors, as listed in Table 1.
TABLE 1 ______________________________________ ELF band VLF band 5 Hz-2 kHz 2 kHz-400 kHz Test Conditions ______________________________________ MPR 25 V/m or less 2.5 V/m or less Temperature 20.degree. C. humidity 21% at a point of 50 cm distant from CRT face TCO 10 V/m or less 1.0 V/m or less Temperature 20.degree. C. humidity 21% at a point of 30 cm distant from CRT face ______________________________________
A treatment for sufficiently shielding an alternating electric field radiated from a deflection yoke is not applied to conventional cathode-ray tube devices. Table 2 lists alternating electric fields [VLF band] on the front of the CRT. Measurement by the inventor reveals that the alternating electric fields (VLF band) depend on horizontal scanning frequencies; as the horizontal scanning frequency rises, the alternating electric fields (VLF band) also increase. Alternating electric fields leaked through the funnel and face panel of the CRT can adversely affect the observer.
TABLE 2 ______________________________________ Charge-resistant 16-inch CRT with coating treatment of 2.6 .times. 10.sup.9 .OMEGA. given to face plate CRT type Charge-resistant CRT Test method MPR TCO ______________________________________ Alternating Horizontal 2.3 V/m 5.0 V/m electric field frequency 31 kHz VLF band (V/m) Horizontal 3.4 V/m 8.3 V/m frequency 45 kHz Horizontal 4.8 V/m 12.0 V/m frequency 64 kHz ______________________________________